Variable transformer
Variable transformers are transformers whose output AC voltage can be adjusted in certain ranges. With outputs of up to a few kilowatts, this is achieved using a grinder ( graphite pencil or roller) that runs on the turns exposed for this purpose (by hand or motorized) and in this way creates a variable number of turns or tap. For larger outputs, special step switches are used for switching .
Variable transformers for three-phase alternating current are known as rotary transformers and consist of a double-fed asynchronous machine in which the rotor is held at a certain angle. The angle determines the transmission ratio.
Layout and function
The voltage is tapped on the exposed turns of the winding with a graphite roller or a graphite sliding contact. The winding voltage must be sufficiently low to limit the losses when two windings are unavoidably bridged by the wiper. The cores of variable transformers are therefore often oversized, which also leads to a lower inrush current. Variable transformers are mostly designed as autotransformers - with these there is no galvanic separation between input and output voltage (economy variable transformer).
Variable transformers are used from low voltages to high-voltage networks and for extra-high voltage transformation, e.g. B. from 220 kV to 380 kV, used to adjust the mains voltage or to set a desired output voltage.
In the past, small autotransformers were used to manually adjust the often strongly fluctuating mains voltage to the fluctuation-sensitive tube devices. In these, the wiper is connected to the mains and the output voltage is taken from a fixed tap on the winding. The adjustment also took place partly with larger steps by means of step switches on taps led out; Here resistors were required for uninterrupted switchover to avoid a short circuit.
In electric locomotives was used prior to the introduction of power semiconductors, a combination of a large variable transformer (main transformer) with large voltage steps and a smaller fine gradation ( "climbing transformer"). The smaller one is used to continuously bridge the steps of the large transformer. This ensures a smooth start-up.
In older analog model railways, variable transformers provide power to the locomotives and their speed setting.
Motor -driven variable transformers are used in mains regulators with higher output, with which mains voltage fluctuations can be compensated for without distortion . Often the output voltage of the variable transformer taken between the slider and a center tap is passed to another transformer: This transforms the current at the expense of the control range and is on the secondary side in series with the controlled output.
Variable transformers with safe mains isolation - so-called isolating variable transformers - are common for laboratory purposes. Devices labeled in this way either contain two transformers (an isolating transformer and an economy variable transformer) or a variable transformer with a safely separated primary winding is used.
literature
- Gerd Fehmel, Horst Flachmann, Otto Mai: The master's examination in electrical machines. 12th edition, Vogel Buchverlag, Oldenburg and Würzburg, 2000, ISBN 3-8023-1795-5
- Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag - Europa - Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3
